[m-rev.] for review: bump the minimum version of Java required

[Julien Fischer]

Hi Peter,

On Fri, 29 Sep 2017, Peter Wang wrote:

> What's the advantage to upgrading? Not that I mind, I'm not using the
> Java backend.

Primarily, so we can make use things of things in the later versions
of Java's library, notably the unsigned integer operations and
java.time package in Java 8.  (It would also allow the work on secure
temporary file creation that's currently sitting in extras to be merged
back into the io module.)

Julien.